MOMENTA PHARMACEUTICALS ADOPTS SHAREHOLDER RIGHTS PLAN

November 9, 2005

Momenta Pharmaceuticals, Inc. (Nasdaq: MNTA), announced today that its Board
of Directors has approved a Rights Agreement pursuant to which the Company's stockholders
will receive preferred stock purchase rights. The Rights Agreement, which is
similar to plans adopted by many public companies, is designed to protect the
long-term value of the stockholders' investment and was not adopted in response
to any current third-party attempt to acquire the Company.